RFID systems are currently used in a variety of applications such as inventory control, theft prevention, access control, etc. RFID systems typically comprise a reader (also referred to as a tag reader or an interrogator) and a tag (also referred to as a transponder).
An RFID tag for use in an inductively coupled RFID system can comprise an integrated circuit (IC) that includes modulation circuitry and non-volatile memory. The tag also comprises an antenna, often referred to as an antenna/coil. RFID tags can be of the active or passive type. Active tags are powered by a battery within the tag. Passive tags derive power to operate by rectifying the alternating magnetic field generated by the reader.
The reader generates a carrier signal in the form of a radio frequency (RF) sine wave. Inductively-coupled RFID systems typically operate with a carrier-signal frequency of approximately 125 kHz or approximately 13.5 MHz. Carrier signals of these frequencies typically are generated by sending an alternating electrical current of a predetermined frequency across a coil-shaped antenna, commonly referred to as a coupler or an antenna/coupler, located in the reader.
The tag becomes inductively coupled to the reader when the tag enters the magnetic field associated with the carrier signal. A transistor in the tag shunts the antenna of the tag when the tag enters the magnetic field and, in the case of a passive tag, when the tag derives sufficient energy from the magnetic field to operate. The shunting of the antenna modulates the carrier wave by damping the amplitude thereof.
The reader continually monitors the carrier wave for modulation. The dampening of the amplitude of the carrier wave is interpreted by the reader as an indication that a tag is present in the magnetic field generated by reader.
The tag can be configured to transmit data to the reader by modulating the carrier wave in a particular manner. For example, the tag can be configured to repeatedly shunt the antenna of the tag so as to encode data in the resulting modulation of the carrier wave. The modulation of the carrier wave can be sensed by the reader, and the data encoded in the carrier wave can be decoded using algorithms in the process of the reader. This technique is commonly referred to as backscatter modulation.
Other types of RFID systems can use propagation coupling as the communication means between the reader and the tag. These types of systems typically operate with a higher carrier-wave frequency, e.g., 2.45 GHz, than inductively-coupled systems. The reader in a propagation-coupled system typically sends an RF signal by way of a dipole antenna. The tag, upon receiving the signal, uses an internal transmitter to send a return signal to the reader at a frequency different than that of the carrier frequency.
The maximum distance across which the reader and the tag can effectively communicate is commonly referred to as the read range of the reader. The strength of the carrier signal is believed to decrease exponentially as the distance between the reader and the tag increases linearly. The read range for an inductively coupled reader and tag is believed to be roughly equivalent to the diameter of the antenna of the reader.
The read range is also related to the relative orientation of the antennas of the reader and the tag. It is believed that the maximum read range can be achieved at a given operating condition when the antennas are substantially parallel. Positioning the antennas in this manner causes the magnetic field generated by the reader to be oriented in a direction substantially parallel to the tag antenna, maximizing the inductive coupling between the reader and the tag. (Read range also can be affected by other factors, such as noise, the quality factor of the reader's antenna and tuning circuit, the modulation and demodulation algorithms, the operating frequency of the reader and tag, etc.)
Readers can be of the hand-held variety. Hand-held readers are relatively small, so that the reader can readily be grasped and carried by the user. The antenna of a hand-held reader usually is incorporated into a common casing with the remainder of the reader's components. This imposes size constraints on the antenna. The read range of hand-held readers therefore is relatively small. The relatively small read range usually requires the user of a hand-held reader to orient the reader so that the antenna is substantially parallel to the tag being interrogated. (Hand-held readers also can be equipped one or more ports for communicatively coupling the reader to an external antenna.)
Hand-held readers often are used in applications where the user moves from item to item to interrogate RFID tags attached thereto. For example, a hand-held reader may be used to interrogate tags attached to palletized stacks of shipping crates or containers located a warehouse, shipping area, transporting vehicle, etc. To achieve maximum read range, the user must align the reader so that the antenna of the reader is substantially parallel to the antenna in the tag being interrogated. This process is repeated as the user approaches each pallet having a tag to be interrogated.
The need to repeatedly position the entire reader at a particular orientation when reading the tags can reduce the efficiency and speed with which the user can progress from tag to tag. Moreover, the location of the tags on the pallets may require the user to hold the reader at an orientation that places the user's hand, arm, etc. in an uncomfortable or unnatural position. This requirement potentially can lead to short-term effects such as muscle fatigue and strain and, over the long term, permanent injuries such as nerve or soft-tissue damage.